1. Field of the Invention
The present invention relates to a color light-emitting diode (LED) driver of an LCD backlight, and more particularly, to a color LED driver, which is capable of being implemented by a compact structure without a feedback structure and accompanying a small size and low cost, by directly connecting a negative temperature coefficient (NTC) thermistor to a driving current path of a color LED applied to an LCD backlight to compensate a characteristic variation of the LED due to a variation in a temperature.
2. Description of the Related Art
Generally, a white LED has been widely used in a mobile device as a light source of a LCD backlight. In a middle-sized or large-sized LCD backlight, a backlight having LEDs of red, green and blue has been developed in order to improve color reproduction. In addition, in order to obtain the same effect, a RGB-LED backlight for a mobile device is being developed.
However, in order to use the RGB LEDs in the mobile device, a light-emitting characteristic deviation according to a temperature needs to be compensated with low cost.
Generally, in a relationship between an ambient temperature and a relative luminance of the LEDs of red, green and blue, when the ambient temperature gradually increases during the operation of the LEDs, light outputs of the RGB LEDs gradually decrease from initial setting values in order of the red LED, the green LED and the blue LED.
However, when the white LED is used in the backlight, the efficiency of the LED decreases as the temperature increases. Accordingly, a luminance decreasing phenomenon occurs, but a color coordinate shift phenomenon hardly occurs. As a result, a temperature compensation circuit is hardly used in the backlight for the mobile device.
In a backlight unit (BLU) using the RGB LEDs, since the luminance decreasing phenomenon and the color coordinate shift phenomenon occur as the ambient temperature increases, the color tends to be shifted to blue, compared with an initial setting state. Accordingly, in the LCD backlight using the RGB LEDs, as described above, a temperature compensation unit for compensating the light outputs of the RGB LEDs which are reduced according to the variation in a temperature and uniformly maintaining the light outputs over time is required, unlike the white LED.
FIG. 1 is a view showing the configuration of a conventional color LED driver.
The conventional color LED driver shown in FIG. 1 includes a driving voltage source 10 for supplying a predetermined driving constant voltage (VD), a driving circuit 20 for converting the driving constant voltage VD of the driving voltage source 10 into red LED driving current Ird, green LED driving current Igd and blue LED driving current Ibd, for driving the color LEDs, and an LED unit 30 including a plurality of color LEDs which are turned on by the red LED driving current Ird, the green LED driving current Igd and the blue LED driving current Ibd from the driving circuit 20.
The LED unit 30 includes a red LED unit 31 including a plurality of red LEDs, a green LED unit 31 including a plurality of green LEDs and a blue LED unit 33 including a plurality of blue LEDs.
In the conventional color LED driver, the brightness (luminance) varies depending on the ambient temperature, due to the LED characteristics. A variation in luminance due to the temperature is shown in FIG. 2.
FIG. 2 is a characteristic graph showing relationships between luminance and temperature of the color LEDs shown in FIG. 1.
Referring to FIG. 2, the luminance of the blue LED hardly varies depending on the variation in the temperature. However, the brightnesses (luminances) of the red LED and the green LED vary depending on the variation in the temperature, because a contact resistance value varies depending on the variation in the ambient temperature and driving current varies depending on the variation in the contact resistance value. Accordingly, the color is shifted to blue.